A core objective of a system for controlling a power electronic device such as a frequency converter is to control output current appropriately for control of an external load, e.g. for regulation of rotation speed of an electric motor, and at the same time in such a way that power components used on the current path are not subjected to excessive strain.
IGBTs are switch type power semiconductor components, which are used widely in main circuit solutions which handle the load current of power electronic devices. An IGBT is a gate-controlled component, meaning that it can be switched to a conducting/non-conducting state by a voltage signal inputted to the gate terminal. An IGBT is a favourable component for power electronic devices, because its fast response time to a control signal enables the control system of a device to control load current with sufficient precision.
The gate voltage controlling an IGBT refers to the voltage between the gate and emitter terminals. Fast control to make the IGBT conduct means that the internal gate capacitance of the IGBT is charged rapidly to a sufficient positive voltage level, and correspondingly, fast control to make the IGBT non-conducting requires rapid release of the capacitance charge to close to zero level. Normally, the gate voltage is controlled to be negative in the standby state; this is unnecessary from the point of view of component control, but increases the margin of certainty against external interference.
It is known that the rapid changes in voltage associated with IGBT connection events can cause interference emissions which move from the device to the environment by conduction and radiation. In the context of long cables, it is known that output voltage pulses with steep edges are reflected at the other end of the cable in accordance with transmission line theory due to a difference in impedance, giving rise to overvoltage spikes which put a strain on motor insulators. Rapid voltage changes also give rise to capacitive current pulses which put a strain on motor bearings.
The connection speed of an IGBT, and therefore also the severity of interference issues as well as so-called connection losses, can be influenced by how fast the gate capacitance is charged and discharged. A direct control circuit of an IGBT, called a gate controller, usually contains at least one positive and at least one negative DC voltage circuit, either one of which is connected to the gate via resistances according to control commands issued by a control unit of the device. The ohm values of these so-called gate resistances can influence the IGBT's connection speed and thereby in turn influence the device's interference levels and internal losses.
Patent publication U.S. Pat. No. 8,558,491 presents a solution in which the ohm value of a gate resistance in use can be changed according to the operating state of an inverter. The operating state is defined on the basis of a measured output current and a measured DC voltage of an intermediate circuit, and the gate resistances of all the IGBTs are changed simultaneously. By means of the solution, the gate resistance value can be optimised at some operating points, but by the same principle, optimisation at many points leads to an unfavourable solution, due to the large number of resistance selecting switches, etc.